D>... выдается следующая ошибка cannot convert parameter 1 from 'struct HWND__ *' to 'struct _RemotableHandle *'
D>Как ее обойти?

typedef union _RemotableHandle switch( long fContext ) u
{
    case WDT_INPROC_CALL:   long   hInproc;
    case WDT_REMOTE_CALL:   long   hRemote;
} RemotableHandle;

typedef [unique] RemotableHandle * wireHWND;
typedef [unique] RemotableHandle * wireHMENU;
typedef [unique] RemotableHandle * wireHACCEL;
typedef [unique] RemotableHandle * wireHBRUSH;
typedef [unique] RemotableHandle * wireHFONT;
typedef [unique] RemotableHandle * wireHDC;
typedef [unique] RemotableHandle * wireHICON;

System handles that are partially remoted 
Partial remoting of a handle means that it has been remoted only as long (or as a DWORD) in both inproc and remote cases. This applies to a handle that would make sense only in the process that generated it, and its remoting as a DWORD was no more than a fix to a legacy problem when some OLE interfaces became remoted. The following handles fall into this category: HWND, HMENU, HACCEL, HBRUSH, HFONT, HDC, and HICON.

All of these are USER handles and behave as described in the section about the GDI and USER handles.

In particular, the following definition has not changed for porting to 64b:

typedef union _RemotableHandle switch( long fContext ) u
{
    case WDT_INPROC_CALL:   long   hInproc;
    case WDT_REMOTE_CALL:   long   hRemote;
} RemotableHandle;
Then the handles are defined in the usual manner:

typedef [unique] RemotableHandle * wireHWND;
typedef [wire_marshal( wireHWND)] void * HWND;

In the future, other branches can be added fairly easily to the union to cover full support for remoting these handles if needed. Please see the section "Changing Data Types Without Changing the Interface Version" to learn more about the meaning of the lack of a default clause in a remoted union.

UA>typedef union _RemotableHandle switch( long fContext ) u
UA>{
UA>    case WDT_INPROC_CALL:   long   hInproc;
UA>    case WDT_REMOTE_CALL:   long   hRemote;
UA>} RemotableHandle;

UA>

typedef struct _RemotableHandle
    {
    long fContext;
    /* [switch_is] */ /* [switch_type] */ union __MIDL_IWinTypes_0009
        {
        /* [case()] */ long hInproc;
        /* [case()] */ long hRemote;
        }     u;
    }     RemotableHandle;

    interface IHDCHolder : IUnknown
    {
        [helpstring("method DrawLine")] HRESULT DrawLine([in] HDC hdc);
        [helpstring("method DrawLine")] HRESULT DrawLineOnWindow([in] HWND hwnd);
    };

STDMETHODIMP CHDCHolder::DrawLine(HDC hdc)
{

    ::MoveToEx(hdc, 10, 10, 0);
    ::LineTo(hdc, 100, 10);

    return S_OK;
}

STDMETHODIMP CHDCHolder::DrawLineOnWindow(HWND hwnd)
{
    HDC hdc = 0;
    hdc = ::GetDC(0);
    
    ::MoveToEx(hdc, 10, 10, 0);
    ::LineTo(hdc, 10, 100);
    
    ::ReleaseDC(0, hdc);

    return S_OK;
}

#include "stdafx.h"
#import "D:\\Test\\dc_test\\Debug\\dc_test.dll" no_namespace

int main(int argc, char* argv[])
{
    ::CoInitialize(0);

    IHDCHolder* hdc_holder = 0;

    ::CoCreateInstance(__uuidof(HDCHolder), 0, CLSCTX_ALL, __uuidof(hdc_holder), reinterpret_cast<void**>(&hdc_holder));

    HDC hdc = 0;
    hdc = ::GetDC(0);

    hdc_holder->DrawLine(reinterpret_cast<RemotableHandle*>(hdc));
    hdc_holder->DrawLineOnWindow(reinterpret_cast<RemotableHandle*>(0));
    
    ::ReleaseDC(0, hdc);
    hdc_holder->Release();


    ::CoUninitialize();
    return 0;
}